The recovery of elemental sulfur from hydrogen sulfide-containing gas streams is known in the prior art as disclosed in the article "Fundamentals of Sulfur Recovered by the Clause Process" by B. Gene Goar, published in the 1977 Gas Conditioning Conference Report.
Oxygen-enrichment in the operation of a Claus sulfur plant to increase the capacity of hydrogen sulfide treated in such a plant has also been disclosed in the article "Oxygen Use in Claus Sulfur Plants" by M. R. Gray and W. Y. Svrcek, published in the 1981 Gas Conditioning Conference Report. It was disclosed more specifically that oxygen can be added to the air feed to the burner of a reaction furnace in a Claus sulfur plant to increase the amount of hydrogen sulfide which is combusted to sulfur dioxide for later catalytic conversion to elemental liquid sulfur product. The maximum capacity increase which can be achieved with oxygen enrichment is determined by the pressure drop through the plant, the reactor space velocity and temperatures of the reaction furnace in the various catalytic zones, particularly the refractory materials used in the furnace of the Claus plant.
In the 1983 publication by Linde of Union Carbide entitled "Claus Plant Oxygen Enrichment", it is noted that oxygen-enrichment limitations exist for rich hydrogen sulfide streams due to temperature limits in the furnace or waste heat boiler of the Claus plant. Oxygen enrichment is expressed as percent oxygen in the oxygen enriched oxidant gas (air). For example if 58 moles of oxygen are mixed with 100 moles of air containing 21 moles of oxygen, the resultant mixture is 158 moles of 50% oxygen enriched air (oxidant gas). This definition of oxygen enrichment still applies if the air and enriching oxygen are not premixed but are instead mixed in the flame or combustion chamber. Oxidant gas is used to contemplate any air and added oxygen fed to the Claus reaction furnace.
U.S. Pat. No. 3,822,341 discloses a Claus plant which uses oxygen-enrichment. One source of the oxygen is initially used to strip residual SO.sub.2 from a sidestream in vessel 92, before the oxygen stream in line 96 is optionally recycled with the oxygen in line 12 going to the combustion zone of the waste heat boiler 8, as recited at col. 5, lines 65-68 of the specification. Because the oxygen content of such a stream is completely consumed in the exothermic reaction, this stream cannot be utilized as a moderating medium for flame temperature of the reaction furnace. As described by the Goar article above, Claus sulfur plants typically have an adiabatic reaction furnace followed by a waste heat boiler. The excessive temperature problem with oxygen enriched operation occurs in the adiabatic reaction furnace. U.S. Pat. No. 3,822,341 ignores the existence of this problem and does not suggest any solution.
U.S. Pat. No. 4,153,674 discloses a Claus plant and tail gas clean up plant wherein a gas stream in line 20 is removed from a tail gas system and is returned or recycled to the front end of the Claus plant 7. This patent does not consider oxygen-enrichment or flame temperature moderation by a recycle stream. Also, a tail gas is reacted to convert all sulfur to hydrogen sulfide, which is absorbed, stripped and returned to the Claus plant.
U.S. Pat. No. 4,212,817 discloses that a liquid diluent (water) may be added to a highly exothermic reaction such as the methanization of carbon oxides via the reversible reactions EQU CO+3H.sub.2 .revreaction.CH.sub.4 +H.sub.2 O EQU CO.sub.2 +4H.sub.2 .revreaction.CH.sub.4 +2H.sub.2 O
to moderate reactor temperatures. The patent teaches that a reaction product additive, water in this case, is desirable so that an equilibrium shift to the left occurs and decreases the extent of the exothermic reaction and the resultant heat release.
U.S. Pat. No. 4,212,855 shows a process for sulfuric acid production wherein a portion of the effluent from the combustion furnace 2 is cooled by direct contact with water and the cooled effluent gases are returned as a temperature moderant to the furnace 2.
U.S. Pat. No. 4,279,882 discloses a sulfur recovery process which uses only a series of catalytic reaction beds rather than a combustion reaction furnace, as in the traditional Claus plant. A temperature modifying recycle stream is set forth in the patent, wherein stream 26 is returned to the feed in order to control the temperature in the catalytic reaction zones. This process is economical only for dilute hydrogen sulfide feed gas applications. It also requires a recycle blower operating at high temperature.
The present invention overcomes the shortcomings of the prior art by increasing throughput of a Claus plant with oxygen-enrichment to an extent beyond that considered feasible in the prior art because of flame temperature limitations. In addition, the present invention provides better throughput of reaction components through the Claus plant reaction train by reducing the carryover of inerts through the system. This is achieved by injecting sulfuric acid into the reaction furnace of the Claus plant. The combination of oxygen enriched oxidant gas and sulfuric acid injection decreases pressure drop in the downstream portion of the Claus plant over an air-only Claus plant operation. The highly endothermic decomposition of the injected sulfuric acid moderates the reaction furnace temperature at high oxygen enrichment levels. Additionally, because the sulfuric acid decomposition produces sulfur dioxide and oxygen, the requirement of expensive oxygen is decreased.